This invention relates to a propulsion system arrangement for a fluidborne vehicle which reduces drag from the viscous boundary layer that is generated by movement of the vehicle through a fluid medium. The viscous boundary layer is inducted into an inlet in the rear portion of the vehicle where the boundary layer fluid is accelerated and then discharged.
Many types of vehicles use propulsion systems which generate thrust on the vehicle by accelerating surrounding fluid. However, most of these propulsion systems are arranged such that fluid from the free-stream surrounding the vehicle approaches the inlet at or near to the speed of the vehicle relative to the far field fluid medium. For most types of gas turbine powered fixed wing aircraft, this is desirable since the high velocity of the fluid approaching the inlet is transformed into pressure head as the fluid is brought to near stagnation at the engine inlet. This reduces the pressure ratio that must be produced by the gas turbine compressor, which translates into a smaller compressor, higher cycle efficiency for the engine and reduced engine weight. The trade-off of course, is increased form, or pressure, drag of the vehicle as a whole, caused by suspending the blunt engine inlet into the free-stream surrounding the vehicle. This almost universal engine arrangement is likely the result of careful trade-off studies performed in concert by aircraft and engine manufacturers to optimize to overall performance of the aircraft; or may simply be a carry-over from reciprocating engine arrangements which do not require large volumes of compressed gas, but are conveniently arranged to drive a propeller rotating in a plane perpendicular to the direction of vehicle motion.
It is also preferable to arrange shaft driven propellers and fans such that the approaching fluid has as near to uniform velocity as possible over the entire inlet area The propeller and fan blade pitch and geometries can be optimized for a narrow range of approach velocities and individual blades will undergo greater cyclic loading, and hence accelerated fatigue and reduced propulsive efficiency, if the fluid velocity upstream of the propeller or fan has a non-uniform angular distribution. This is another possible motivation for the placing of aircraft propellers and fans out away from the boundary layers generated around lifting surfaces, control surfaces and fuselages.
A number of experimental and commercial aircraft have implemented various departures from the forward facing propeller/fan/engine inlet suspended into the freestream. Rotary-wing aircraft for instance, commonly use radially oriented engine inlets, although the main rotor system and tail rotor are almost always located as far away from the main fuselage as practical and as such do not make use of boundary layer flow. So called "pusher-prop" aircraft use propellers or fans at the rear of one or more fuselage or sponson structures, drawing fluid from the boundary layer surrounding the fuselage or sponson to some extent, but the main portion of the fluid acted on by such propellers is generally accepted to be free-stream flow. Any benefit of removing boundary layer flow in these configurations is negated by the propeller wake i.e., "prop wash" which is larger in diameter than the wake from the fuselage or sponson alone and by the fact that the contribution to overall form drag from the fuselage or sponsor boundary layer is small when compared with that generated by the large lifting surfaces.
Experimental aircraft such as the X-21A constructed and tested by the National Aeronautics and Space Administration (NASA) Jet Propulsion Laboratory (JPL) have used a sophisticated arrangement of fans, ducts and vent holes distributed over the entire outer surface of the aircraft to almost completely remove all boundary layer fluid. Although the aircraft proved successful from an aerodynamic standpoint, reducing form drag by 20-30%, the complex ducting and exhauster fans left no weight margin for any effective payload and the thousands of small vent holes were impractical from a maintenance standpoint. The holes become almost completely blocked after only a few test flights even in relatively clean runway conditions.
Many other types of fluidborne vehicles use propulsion systems that accelerate fluid surrounding the vehicle but use inlets which face directly forward, drawing fluid only partially from the boundary layer generated by the main body of the vehicle or not at all. Vehicles such as airships have no lifting surfaces and relatively small control surfaces. As such, the form drag of an air ship is due almost entirely to momentum losses in the wake generated by the fluid boundary layer which forms around the main body of the vehicle. Small unmanned aircraft also typically have almost nonexistent lifting surfaces and as such, could benefit in terms of reduced form drag if the conventional forward facing engine inlet cowling were replaced with a more conformal engine inlet designed to remove as much fluid as possible from the boundary layer formed around the aft portion of the aircraft's fuselage.
Seagoing vessels have recently begun using water jets in larger numbers which have inlets that are directed more toward the sides of the hull rather than the conventional propeller drawing fluid from what is typically a complex, swirling mixture of boundary layer fluid moving along the bottom of the hull and fluid from the surrounding free-stream. However, conventional water jet propulsion systems currently available for seagoing vessels have small inlets capable of removing boundary layer fluid from only a small portion of the vessel's girth below the waterline. Hence, the majority of the boundary layer flow continues aft only to be released into the turbulent wake behind the ship, where the kinetic energy added to this fluid when it was accelerated nearly to the vessels forward speed is gradually dissipated in swirling eddies.
Small submersible vehicles, both manned and unmanned have been developed which use propulsion means with a large degree of boundary layer ingestion, but are accompanied by after body shapes that taper rapidly causing flow separation during under-thrusted, i.e., under deceleration conditions. This flow separation is tolerable for vehicles which have control surfaces in the free stream or forward of the inlet to the propulsion means which can maintain stable attitude control over the vehicle during under-thrusted conditions. However, the preferred location for such control surfaces is in the aftmost section of the vehicle where the greatest control can be applied with the smallest control surface.